The point of SD3's six character setup is that you choose who the main character is. The story (well, the dungeons and villains) are then patterned around the character. For instance, if you choose Kevin, the final third of the game is dedicated to Deathjester, Heath, and the Masked Mage, who goes on to threaten the Goddess. In this timeline, the other villains are already dead, and therefore it would make no sense to say that, for instance, Duran was the plot's main character, whether he's in the party or not.